Potential of an Induction- Coil at Ci make." 32& 



especially in X-ray work, it is desirable to suppress the 

 " inverse current " at make; and even i£ one of the usual 

 rectifying devices is employed for the purpose — most of 

 these are said to be but imperfect rectifiers — it is still im- 

 portant to reduce as far as possible the maximum potential 

 to which the secondary terminals rise after contact is made 

 at the interrupter. It need hardly be said, however, that 

 in considering the various methods by which the make- 

 potential may be reduced, the importance should be borne in 

 mind of maintaining undiminished the potential at break. 

 In the following calculation we shall regard the secondary 

 circuit as open, and endeavour to find an expression for the 

 make-potential in terms of the various coefficients of the two 

 circuits. 



Denoting the primary and secondary currents by t l9 i 2T 

 the coefficient of induction of the primary coil on the 

 secondary by L 2l , that of the secondary on the primary 

 by L 12 , and with the usual notation for the other quantities, 

 the equations are 



L *§+ L 4 +H ^ E ' w 



L s ^ + L ai ^+B*t, + V,=0, ... (2) 

 at at 



*=<^: . . (3) 



It will be noticed that there is in these equations no term 

 depending upon the capacity of the primary condenser. 

 Although there is a very prevalent belief that the induced 

 current in the secondary at make depends upon this capacity, 

 which is said to " take up and store " the first rush of current, 

 thus retarding the growth of current in the primary coil, it is 

 difficult to understand how any such action can take place. 

 At make, the condenser becomes short-circuited and, after 

 losingfits initial charge CiE — none of which can pass through 

 the coil — takes no further part in the proceedings. Expe- 

 riment also shows that the primary condenser does not assist 

 in reducing the secondary spark-length at make — at least 

 this is certainly the case when the interval between break 

 and make is not extremely short, so that the oscillations set 

 up at break have time to die away before the following 

 make. The condenser might indeed be expected to exert 



* The secondary circuit being open and not connected with a condenser 

 the current in it is not uniformly distributed along- the wire. * a is the 

 current in the central winding, and V 2 is the P.D. at the secondary 

 terminals (see Phil Mag. April 1914, p. 066). 



